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Control Mechanisms of Secretion of Classical HormonesBY: Aritri Ghosh | Category: Biology | Submitted: 2011-05-18 11:35:28
Hormones are biochemical compounds that activate a reaction or biological function. They control in a way every action in our body from growth to birth. They are essential for proper physical and mental development. While some hormone control the metabolism and other biological functions, some control the development process and proper maintaining of our body. For the hormones to work properly for a healthy and fully developed human body the hormones needs to be controlled so that they are secreted in the right place, in required quantity and for proper function. The control mechanisms of hormones are described in brief below.
Secretions of the classical hormones are controlled by the following mechanisms:
1. Hypothalamic Control
2. Anterior Pituitary Control
3. Feedback Control
4. Stress Control
5. Circadian Rhythm
6. Neural Control
Hypothalamus is situated in the under surface of the brain. It produces four releasing hormones and two inhibiting hormones. The releasing hormones include
They stimulate tropic and other hormones of the anteriorpituitary. For example CRH or corticotrophin releasing hormone is produced by the hypothalamus. CRH stimulates the cells called corticotrophs which produce corticotrophin hormones also called ACTH. ACTH secretion is stimulated by this.
Anterior Pituitary Control
Anterior pituitary produces five tropic hormones named somatotrophin or growth hormone, corticotrophin or ACTH, thyrotrophin or thyrotrophic hormones or TSH-thyroid stimulating hormones, prolactin and gonadotropin. Cells producing these tropic hormones are under various controls one of which is hypothalamic control mentioned before.
These tropic hormones after being secreted stimulate their target organs. The examples are ACTH or corticotrophin stimulates adrenal cortex or to produce cortisone. For all this reasons anterior pituitary is called master for leader of the endocrine orchestra.
Feed Back Control
There are two major varieties of feedback controls in endocrinology.
1. Negative feedback control
2. Positive feedback control
Negative feedback control is most widely present. CRH from hypothalamus stimulates ACTH of anterior pituitary. ACTH stimulates secretion cortisone from adrenal cortex. This lead to an increase in plasma cortisol level. Cortisol inhibits both ACTH secretion and CRH secretion. Thus excess cortisol secretion is inhibited and plasma level of cortisol becomes normal again.
On the other hand if plasma cortisol level is low or lower than the normal CRH and ACTH become freed of the inhibitory influences of cortisol, more CRH and ACTH is released. This leads to production of more cortisol. Thus cortisol levels in plasma become normal again.
The hormones secreted from hypothalamus and pituitary are the hormones which are normally controlled by feedback mechanisms. The hormones from hypothalamus are negative feedback controlled by the pituitary hormones.
In short stress destabilizes or threatens to destabilize the homeostasis and if not checked may lead to death. Common examples of stress include mental like acute apprehension or death of self or death of a near and dear one, physical like street accidents or high fever or burn or cardiac diseases and so forth.
During stress several hormones are secreted in increased quantity. This is called endocrine response to stress. This endocrine response to stress tries to minimize the harmful effects of stress. For example, during stress both ACTH and cortisol secretions are increased and continue to increase despite a high level of plasma cortisol level. That is ACTH secretion is stimulated by stress overriding the negative feedback effects of plasma cortisol.
Control by Biological Clock or Circadian Rhythms
Normally the plasma level of cortisol is least in morning and highest in the evening as if within our body there is a clock-the Biological Clock. It dictates the ups and downs of the plasma level of cortisol in a 24 hour cycle.
When a single peak and a single nadir are seen in a 24 hour cycle than the rhythm of the biological clock is said to be circadian rhythm. That is circadian rhythm is a rhythm which shows a periodicity of 24 hour. In circhoral rhythm periodicity is around an hour. Circhoral rhythm in endocrinology is also known. The biological clock is controlled by the hypothalamus area of the brain.
When a man becomes panicky or tensed his sympathetic system is stimulated. The sympathetic nerves in turn stimulate adrenal medulla which secretes adrenalin. Adrenalin enters blood and functions as a classic hormone.
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